This invention relates to the improvement of substrate alignment, for example in the manufacture of semiconductor circuits.
The manufacture of semiconductor circuits requires that patterned layers be aligned to specific underlying features. The amount of misalignment is readily measurable, using commercially available instruments and procedures. It is desirable to minimize misalignment.
As is well known, photolithography methods have been used for patterning a resist layer on a semiconductor wafer to form integrated circuits (ICs) such as processors, ASICs and Dynamic Random Access Memory (DRAM). As dimensions of features within the ICs on the semiconductor wafer become smaller, repeatable accurate photolithography methods become more important.
Presently, one of the most conventional photo lithography methods used is a stepper, a step-and-repeater, which moves and aligns a wafer based on alignment marks on a reticle containing an image such that desired patterns on the wafer are exposed based on the image. The stepper includes a radiation lamp for generating radiation, an imaging lens, a reticle stage for mounting and moving the reticle and a wafer stage for loading the wafer. In the stepper, after the wafer is loaded on the wafer stage, the reticle stage searches and moves to a predetermined position to form a chip, thereby implementing an alignment process. Thereafter, amounts of misalignment errors such as X misregistration, Y misregistration, rotation and orthogonal errors are measured to accurately position the reticle stage at the predetermined position. In the next step, the radiation from the radiation lamp propagates to the imaging lens after passing through the reticle. The wafer is exposed by the radiation from the imaging lens by moving the wafer stage until the entire of the wafer is scanned. A number of successive steps of photolithography, film growth, deposition and implantation of impurities create a complete IC with many identical copies or chips on a same wafer.
Prior art exposure tools, such as a stepper, typically allow measured corrections to be entered so that alignment is improved on subsequent wafers. For example, a measured error is entered as a correction and the stepper is instructed to correct subsequent wafers by the measured inaccuracy. As such, stepper alignment corrections are typically made by measuring the alignment and reversing the measured component offsets. For example, depending on conventions used for a particular stepper or lithographic layer, engineers know to input the measured parameter or to reverse the sign.
There exists a problem in that one correction can interact with another, and also the response to a correction may be some fraction of the actual programmed value.
Broadly, the invention is to a method for improving alignment of a substrate on a stepper, comprising the steps of imposing predetermined corrections for each of a plurality of substrates, a set of corrections for each substrate, measuring the actual corrections which occur on each substrate, mathematically processing the actual corrections to produce a matrix and provide a plurality of equations, and resolving the equations to provide a set of input corrections to provide a correct alignment.